1. Field of the Invention
The present invention relates to a motor control apparatus in which AC power supplied from an AC power supply side is converted to DC power to be output, and then the converted DC power is further converted to AC power for driving a motor in order to supply the power to the motor. More particularly, the invention relates to the motor control apparatus with a power failure determination unit for determining presence or absence of a power failure at the AC power supply side.
2. Description of the Related Art
In a motor control apparatus for driving motors used in machine tools, forming machines, injection molding machines, industrial machines, or various kinds of robots, DC power temporarily converted from AC power which is input from an AC power supply side is further converted to AC power. The converted AC power is used as drive power for a motor provided for each of drive axis. The motor control apparatus includes: a rectifier for outputting DC power by rectifying AC power supplied from an AC power supply side provided with a three-phase AC input power supply; and an inverter, being connected to a DC link at a DC side of the rectifier, for performing a bidirectional power conversion between DC power at the DC link and AC power being drive power for a motor or regenerative power. The motor control apparatus controls the speed, torque, or position of a rotor of the motor connected to the AC side of the inverter.
In such a motor control apparatus, when an input power supply voltage decreases due to power failure at an AC power supply side of a rectifier, it is difficult to continue a normal operation of the motor. Therefore, faults such as damage and deformation may occur to: the motor; a motor control apparatus for driving the motor; a tool connected to the motor driven by the motor control apparatus; a processing target to be processed by the tool; a manufacturing line with the motor control apparatus; and the like. Therefore, with a power failure determination unit being provided at an AC power supply side of the rectifier and with a presence or absence of a power failure of the AC power supply side of the rectifier being monitored, it is necessary that the motor control apparatus operates to perform a protective operation in order to avoid or minimize the faults described above, when the power failure determination unit determines that a power failure has occurred at the AC power supply side of the rectifier.
As a power failure determination method, there is a method of calculating an amplitude value of a power supply voltage by coordinate conversion of a three-phase AC input voltage at an AC power supply side of a rectifier to a voltage vector equivalent thereto on a two-phase coordinate and calculating an amplitude value of the vector; and then detecting a power failure when a state in which the calculated amplitude value is less than a predetermined reference voltage value has continued for a predetermined reference period, as described in Japanese Laid-open Patent Publication No. 2006-14546, for example. FIG. 9 is a diagram illustrating a power failure detection method in an invention described in Japanese Laid-open Patent Publication No. 2006-14546. AC voltage detection unit 111 detects an AC voltage at a side to which three-phase AC input power supply 3 of a rectifier 100 constituting a motor control apparatus is connected and a voltage amplitude calculation unit 112 calculates a voltage amplitude value thereof. A power failure determination unit 113 determines that a power failure has occurred at the AC power supply side of the rectifier 100 when a state in which the amplitude value calculated by the voltage amplitude calculation unit 112 is less than a predetermined reference voltage value has continued for a predetermined reference period.
Furthermore, in recent years, a rectifier using pulse width modulation (PMW) (hereinafter, referred to as the “PMW rectifier”) has been widely utilized as a rectifier in a motor control apparatus due to a request for reducing power supply harmonics and reactive power. The PWM rectifier is configured as a bridge circuit of semiconductor switching elements, and performs a power conversion operation to convert input AC power to DC power to output the converted DC power, by controlling a switching operation of the semiconductor switching elements by a switching command.
For example, as described in Japanese Laid-open Patent Publication No. H7-322626, in a motor control apparatus with a PWM rectifier, there is a method for determining that a power failure has occurred when the apparatus detects that a PWM frequency component is included in a signal, a level of which changing in accordance with the polarity of a power supply voltage, and which includes the same frequency component as the power supply frequency.
Furthermore, for example, as described in Japanese Laid-open Patent Publication No. 2009-44781, there is a method for detecting a power failure when an absolute value of a voltage command value in a motor control apparatus being equal to or less than a power failure detection level.
However, in a case that a PWM rectifier is applied to a rectifier described in Japanese Laid-open Patent Publication No. 2006-14546, a power failure at an AC power supply side may not be detected in the method described above during a regenerative operation in which a DC output voltage does not decrease since the DC output voltage appears as an input voltage through the rectifier circuit, when a power failure has occurred at the AC power supply side of the PWM rectifier which is an open state. FIG. 10 is a diagram illustrating a power failure detection at an AC power supply side of a PWM rectifier during a regenerative operation in which a DC output voltage does not decrease, in a motor control apparatus with a conventional PWM rectifier and an inverter. Since an on-off control of switching elements is always performed in a PWM rectifier 200, a DC voltage appears at an AC voltage detection unit 211 through switching elements which is in an on state as illustrated in FIG. 10 when an AC power supply side of the PWM rectifier 200 becomes an open state during an occurrence of a power failure. When the PWM rectifier 200 is in a powering operation in which AC power is converted to DC power, a power failure determination unit 213 can perform a power failure detection by a voltage amplitude because a DC voltage output from the PWM rectifier 200 decreases, an AC voltage detected by the AC voltage detection unit 211 also decreases, and therefore a voltage amplitude value calculated by a voltage amplitude calculation unit 212 decreases, although there are some delays in detection. However, when the PWM rectifier 200 is in a regenerative operation in which DC power is converted to AC power, the power failure determination unit 213 may not detect a power failure at the AC power supply side of the PWM rectifier 200 because a DC voltage increases due to regenerative energy from a motor (not illustrated), an AC voltage detected by the AC voltage detection unit 211 also increases, and a voltage amplitude value calculated by the voltage amplitude calculation unit 212 increases.
In addition, according to an invention described in Japanese Laid-open Patent Publication No. H7-322626, in a signal having the same frequency component as a power supply frequency used for determining a power failure occurrence, a PWM frequency component is less likely to appear due to LC filters generally provided at an AC power supply side of a PWM rectifier. Therefore, there is a drawback that a power failure can be stably detected only when there are no LC filters.
Furthermore, according to an invention described in Japanese Laid-open Patent Publication No. 2009-44781, it is not possible to detect a power failure when a power failure occurs during a regenerative operation since there is no possibility that a voltage command value decreases.